Documentation: networking: phy.txt: MDIO bus reset is optional

Update the MDIO bus documentation to mention that the MDIO bus reset
function is completely optional. It became optional with commit
e1393456 ("[PATCH] PHY Layer fixup")
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/phy.txt

@@ -48,7 +48,7 @@ The MDIO bus
    time, so it is safe for them to block, waiting for an interrupt to signal
    the operation is complete
  
- 2) A reset function is necessary.  This is used to return the bus to an
+ 2) A reset function is optional.  This is used to return the bus to an
    initialized state.
 
  3) A probe function is needed.  This function should set up anything the bus